As energy source prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future life. Thus, research into various power generation techniques such as nuclear energy, solar energy, wind energy, tidal power, and the like, continues to be underway, and power storage devices for more efficient use of the generated energy are also drawing much attention.
In particular, demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and demand therefor continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized and the market for lithium secondary batteries continues to expand to applications such as auxiliary power suppliers through smart-grid technology.
As cathode active materials for lithium secondary batteries, lithium cobalt-based oxides, lithium manganese-based oxides, lithium nickel-based oxides, lithium composite oxides, and the like are mainly used, and carbonaceous materials are mainly used as anode active materials. In addition, research into anode materials, prepared by alloying Li with silicon (Si) or tin (Sn), and lithium titanium oxides is underway.
LiMn2O4 having a spinel structure is an alternative to LiCoO2, which is difficult to be used in high-voltage application in terms of energy density. However, Mn3+ ions in the crystal lattice of the lithium manganese oxide react with an electrolyte at a voltage of 4 V or higher and thus impurities are formed on an electrode surface, and the crystal structure of the lithium manganese oxide is changed at room temperature due to Jahn-Teller distortion and thus reversible intercalation and deintercalation of lithium ions are difficult to implement, which results in dramatic reduction in capacity.
To address these problems, research into a method of substituting an Mn3+ ion of LiMn2O4, which is situated at an octahedral 16d site, with one of the various transition metals (e.g., Cr, Ni, Fe, Cu, and Co), in order to improve properties of LiMn2O4, is underway. Among Mn3+ ion-substituted lithium manganese oxides, LiNi0.5Mn1.5O4 is known that Mn3+ ions can theoretically be effectively inhibited by being substituted with Ni2+ ions and thus enhanced electrochemical properties are anticipated.
However, LiNi0.5Mn1.5O4 has an operating voltage of 4.5 V or higher, and thus, as an electrolyte including the same is oxidized, gas is discharged and by-products are generated, which results in deteriorated battery performance and increased resistance. Consequently, severe problems may occur in terms of battery safety.
Meanwhile, lithium titanium oxides have initial charge and discharge cycle efficiency that approximates to 100% and have a high operating voltage, thus, film formation on a surface of an anode due to electrolyte decomposition reaction does not occur. Accordingly, these lithium titanium oxides are expected to be used as a high-output anode material.
However, such lithium titanium oxide absorbs moisture in air. In addition, a diffusion rate of lithium ions of the lithium titanium oxide is low and thus lithium titanium oxide particles need to be prepared at nanoscale levels in order to decrease the migration distance of lithium ions. However, such nanoscale particles are susceptible to moisture.
Absorbed moisture is decomposed to generate a large amount of gas. Such gas is a cause of battery performance deterioration.